Continuous process for producing pseudoinones and ionones

ABSTRACT

The invention relates to a continuous process for producing pseudoionones of general formulas (I) and (I′) as well as isomers thereof, whereby: R 1  represents CH 3  or (a); R 2  and R 3  represent hydrogen, CH 3  or C 2 H 5 , and; R 4  and R 5  represent hydrogen or CH 3 . These pseudoionones are produced by reacting an aldehyde of formula (II) with an excess of a ketone of general formula (III), whereby R 1 , R 2  and R 3  have the aforementioned meanings, in the presence of water and alkali hydroxide at an increased temperature and in a homogeneous solution. The inventive process is characterized in that: a) the intermixing of the homogeneous solution consisting of aldehyde, ketone and aqueous alkali lye occurs at a temperature ranging from 10 to 120° C.; b) the undissolved water and alkali hydroxide contained in the reaction mixture are subsequently separated out; c) while avoiding back mixing, the homogeneous reaction mixture is then guided through a reactor, which permits a residence time ranging from 2 to 300 minutes, at a temperature that is 10 to 120° C. higher than the boiling point of the lowest-boiling component and under a vapor pressure p ranging from 10 6  to 10 7  Pa; d) the reaction mixture is cooled by expansion; e) ketone is removed from the reaction mixture using vapor flowing in the opposite direction and; f) the raw product is dried and rid from excessive aldehyde and secondary components via a rectification column.

The present invention relates to a continuous process for preparingpseudoionones and the optional subsequent cyclization thereof toionones. Preference is given to the process for preparing compounds suchas 6-methylhepta-3,5-dien-2-one, pseudoionone, methylpseudoionone,dimethylpseudoionone, pseudoirone, methylpseudoirone anddimethylpseudoirone and the cyclization products thereof, α-, β- andγ-ionone, α-, β- and γ-methylionone (n-form, iso-form or mixtures), andhomologs. These substances are of great economic significance asodorants and odorant intermediates. Pseudoionone itself is additionallya significant intermediate for the preparation of vitamins E and A andof carotinoids. β-lonone is a significant intermediate for thepreparation of vitamin A and carotinoids.

For the preparation of pseudoionones from citral, numerous methods areknown.

PL 147748 describes a process for preparing ionones by condensing citraland acetone over basic ion exchangers at 56° C. According to this,acetone and citral are stirred with the catalyst in a flask batchwisefor 5 hours. A disadvantage of this process is the very low space-timeyields.

DE-A 33 19430 teaches the preparation of higher ketones by condensingmethyl ketones and unsaturated aldehydes over mixed metal catalysts inthe presence of hydrogen at from 100 to 280° C. and from 10 to 60 bar ina tubular reactor.

A process for preparing pseudoionones by reacting citral with acetoneusing LiOH as a catalyst is described in U.S. Pat. No. 4,874,900.According to this, the reaction is carried out batchwise or continuouslyat temperatures of from −20 to 240° C. The pressure is adjusted suchthat the reaction mixture remains in the liquid phase at the appropriatetemperature. In the case of batchwise operation, the reactants arestirred in a tank and the catalyst is filtered off on completion of thereaction, while, in the continuous mode, the premixed reactants arepumped through a column filled with catalyst. In both cases, thereaction mixture is neutralized with CO₂ after the end of the reactionand the excess ketone is distilled off. In this process, at a molaracetone to citral ratio of 20, yields of 89.5% citral are obtained.These low yields are unsatisfactory for an industrial scale process.

DE-A 31 14071 describes a process for preparing pseudionones by reactingan aldehyde with an excess of a ketone at elevated temperature.

The prior art also discloses numerous methods for the subsequentcyclization of pseudoionones to ionones. For instance, it is known thatmixtures of α- and β-ionones are obtained in the cyclization ofpseudoionones with acids such as concentrated sulfuric acid orphosphoric acid. The ratio of the amounts in which these compounds areformed depends greatly upon the conditions under which the reactiontakes place.

In cyclizations with concentrated sulfuric acid, which proceed highlyexothermically, it is important to remove the heat of reaction veryrapidly in order to prevent localized hotspots. For this purpose,diluents are added to the reaction mixture in the known processes.

It is an object of the present invention to develop a process forpreparing pseudoionones and for cyclization to the corresponding iononeswhich follows if appropriate, which needs fewer feedstocks in comparisonto the prior art and generates more product per feedstock.

1. According to the invention, the object is achieved by providing acontinuous process for preparing pseudoionones of the general formulae Ior I′ and isomers thereof

-   -   where R¹ is    -   R², R³ are each hydrogen, CH₃ or C₂H₅,    -   R⁴, R⁵ are each hydrogen or CH₃,    -   by reacting an aldehyde of the formula (II)    -   with an excess of a ketone of the general formula (III)    -   where R¹, R² and R³ are each as defined above, in the presence        of water and alkali metal hydroxide at elevated temperature in        homogeneous solution, which comprises    -   a) mixing the homogeneous solution of aldehyde, ketone and        aqueous alkali metal hydroxide at a temperature of from 10 to        120° C., then    -   b) removing the water and alkali metal hydroxide which have not        dissolved in the reaction mixture,    -   c) subsequently passing the homogeneous reaction mixture,        avoiding backmixing, at a temperature which is from 10 to        120° C. above the boiling point of the lowest-boiling component        and a vapor pressure p of from 10⁶ to 10⁷ Pa through a reactor        which enables a residence time of from 2 to 300 minutes,    -   d) cooling the reaction mixture under decompression,    -   e) removing the ketone from the reaction mixture with steam in        countercurrent and    -   f) drying the crude product and freeing it of excess aldehyde        and secondary components using a rectification column.

Preference is given to using the process according to the invention forpreparing 6-methylhepta-3,5-dien-2-one, pseudoionone,methylpseudoionone, dimethylpseudoionone, pseudoirone, methylpseudoironeand dimethylpseudoirone and isomers thereof.

The aldehydes used in accordance with the invention are preferablycitral, citronellal and 2,6-dimethyloctanal, but also any straight-chainor branched, saturated or else unsaturated, aldehyde having from 1 to 10carbon atoms, and the ketones used are preferably acetone, 2-butanone,or 2- or 3-pentanone.

Aqueous alkali metal hydroxide refers to an aqueous solution ofpotassium hydroxide, sodium hydroxide or lithium hydroxide, butpreferably sodium hydroxide solution. The concentration of the alkalimetal hydroxide used is between 0.005 and 50% by weight, preferablybetween 1 and 15% by weight.

The isomers refer to all possible positional isomers or double bondisomers of the pseudoionones or ionones.

In the process according to the invention, only as much alkali metalhydroxide solution is added at from 10 to 120° C., preferably attemperatures of less than 50° C., to the homogeneous mixture of thealdehyde, ketone and water reactants as dissolves homogeneously afterintimate mixing. Any water and alkali metal hydroxide which separates isremoved, before the homogeneous reaction mixture is passed, whileavoiding backmixing, at a temperature which is from 10 to 120° C. abovethe boiling point of the lowest-boiling component and a pressure p offrom 10⁶ to 10⁷ Pa where p is the vapor pressure of the reaction mixtureat the reaction temperature, through a reactor which enables a residencetime of from 2 to 300 minutes, preferably from 5 to 30 minutes. Thereaction mixture is cooled by decompression, in the course of which aportion of the ketone excess evaporates and can be fed to recycling,then the ketone is removed from the reaction mixture with steam incountercurrent, the steam containing enough of an evaporable acid thatthe catalyst base is neutralized, and a pH of from 4 to 9 isestablished. Subsequently, the crude product is dried and freed ofexcess aldehyde and secondary components using a rectification column,preferably using a dividing wall column, as disclosed, for example, inDE-A 3302525 or in EP-A 804 951.

The invention further provides a continuous process for preparingionones of the general formulae (IV), (V) and (VI) and isomers thereof,which comprises converting the pseudoionones obtained by the processaccording to the invention to ionones of the general formulae (IV)-(VI)

It is surprising that the formation of secondary and decompositionproducts which arises as a side reaction in the heterogeneous catalysisby alkali metal hydroxide, in particular the workup of the reactionmixture, can be suppressed when the mixture of ketone and aldehyde isadmixed below the process temperature in the reactor only with as muchalkali metal hydroxide solution as can be dissolved homogeneously, andthe homogeneous mixture saturated with aqueous alkali metal hydroxidesolution is brought to the desired reaction temperature under autogenouspressure in a tubular reactor without further mixing.

It is advantageous to remove at the reactor inlet any alkali metalhydroxide solution occurring which has not dissolved into the mixtureand is thus excess. This may be effected in a separator which is eitherattached upstream of the reactor or is integrated into the bottom of thereactor. It is also advantageously possible to remove excess water fromthe ketone to be recycled by metering into the reaction mixture highlyconcentrated, i.e. from about 10 to 50%, preferably from 35 to 45%,alkali metal hydroxide solution, which dehydrates the reaction mixtureand dissolves the required amount of alkali metal hydroxide into thereaction mixture.

The reaction is conducted with a from 5-50-fold, preferably with a from20-25-fold, molar excess of ketone in order to achieve an optimal yieldbased on the aldehyde used. The unconverted ketone fraction is removeddownstream of the reaction zone at a pressure of from 10⁷ to 10⁹mPa_(abs) and fed back to the fresh ketone for the synthesis.

Surprisingly, the water content of the aldehyde-ketone mixture is alsoof particular significance. This apparently determines the amount ofalkali metal hydroxide which can dissolved homogeneously in thealdehyde-ketone mixture. The water content of the aldehyde-ketonemixture should be between 1 and 15% by weight. The amount of alkalimetal hydroxide dissolved in turn determines the conversion rate, butalso influences the occurrence of undesired by-products. This is inagreement with the fact that the removal of excess alkali upstream ofthe reactor is advantageous. In contrast to the prior art, this preventsfurther dissolution of alkali metal hydroxide into the reaction mixturetoward the end of the reaction owing to the increase in the watercontent due to the water of reaction, which promotes by-productformation in this phase. The latter plays a significant role inparticular in the case of sensitive unsaturated aldehydes, for examplecitral, and reduces the yield.

The water is advantageously introduced into the process via theproportion which is within the ketone component and is generateddownstream of the reactor by the steam stripping of the reactionmixture. It is of economic significance that this allows the ketoneexcess to be removed with low technical complexity and energy intensity,since the complicated drying before the recycling becomes superfluous.Alternatively, it is also possible to proceed with an anhydrous mixtureof aldehyde and ketone and mix in the water required (from about 1 to15% by weight), by using a very dilute alkali metal hydroxide solution.Conversely, it is possible to use a mixture of aldehyde and ketonehaving a very high content of water when a concentrated alkali metalhydroxide solution is mixed in. In this case, a lower mixing temperatureis required in order to prevent the uncontrolled start of the reaction.At the same time, the consumption of alkali metal hydroxide rises, sinceit is only partly transferred into the organic phase. It partlydehydrates the aldehyde-ketone mixture and has to be removed anddisposed of.

The homogeneous reaction solution is heated in a tubular reactor underautogenous pressure, and the reaction temperature at a given residencetime is adjusted such that the conversion of the aldehyde component isfrom 60 to 98%, preferably from 85 to 95%, and the unconverted aldehydeis removed and recycled into the reaction. The tubular reactor isdimensioned such that the average residence time is between 2 and 300minutes, preferably between 5 and 30 minutes, in such a way that thereis very little backmixing. Higher conversions entail disproportionatelyraising the reaction temperature, which promotes by-product formation.Lower conversions enable a lower reaction temperature, which suppressesthe by-product occurrence, but the ketone and aldehyde recycle streams,and thus the energy demands of the process, increase.

In the tubular reactor, the backmixing has to be minimized. This can beachieved by a sufficiently large reactor diameter in order to avoidturbulence, or else also by laminar-flow internals of any type. This issurprising and is in contradiction to the prior art, where, for example,according to DE-A 31 14 071, tubular reactors have to have such a designthat there is a sufficiently turbulent flow under the reactionconditions.

The reaction mixture is decompressed to standard pressure, in the courseof which it cools via the evaporation of a portion of the excess ketone.The remaining ketone is driven out in a countercurrent column with steamto which has been added an equimolar amount of a volatile acid, in thecourse of which the catalyst base is neutralized and diluted by thecondensate. The use of column packings ensures that no significantamounts of further products in addition to ketone and water are obtainedat the top of the column, and the reflux to the column is advantageouslyadjusted in such a way that the ketone can be drawn off with the desiredamount of water. The amount of acid is advantageously such that the pHof from 4 to 9 favorable for the further workup is established at thispoint. After removal of the aqueous phase, the crude product is dried,by heating it and spraying it into a flash vessel which is kept underreduced pressure. From there, the mixture is transferred into arectification column in which the unsaturated ketone is purified underreduced pressure to free it of impurities and the unconverted aldehydeis removed and fed from there to the recycling. The recycling iseffected advantageously in a dividing wall column as described in EP-A804 951, which preferably has 2 side draws in order to obtain both mainfractions (product and aldehyde) in sufficient purity in one step.

The above-described process has very particular significance when citralis used as the aldehyde component and 2-butanone as the ketonecomponent. A characteristic mixture of 70-97% n-methylpseudoionone and3-30% isomethylpseudoionone is formed and can be cyclized to give acharacteristic mixture of methylionones. Methylionones exist in highlyvarying isomer ratios. Each of them is used by the odorants industry ina large amount for producing industrial perfumes. Since each isomerratio has a somewhat different fragrance note, the reproducibility of anisomer ratio once used is of utmost importance.

To prepare the corresponding ionones, the resulting mixture ofpseudoionones is reacted with highly concentrated, i.e. from about 50 toabout 98%, sulfuric acid, in the presence of a diluent which is inertunder the reaction conditions, more advantageously as described in DE196 19 557. In a departure therefrom, cyclization temperatures of <20°C. and a sulfuric acid concentration of <90% are advantageous when aresidence time of >10 seconds is maintained between cyclization andhydrolysis. In the case of methylionones, a sulfuric acid concentrationof about 89% at a residence of about 2 min at about 25° C. isadvantageous, in which case almost exclusively the β-isomers areobtained, while the formation of α-n- and γ-n-methylionones issuppressed to about 1%. In the case of pseudoionone, virtuallyexclusively β-ionone is formed in high yield with 89% sulfuric acid,while α-ionone and γ-ionone are only within the 1% range and can beremoved overhead in the purifying distillation without any problem.

The examples which follow will illustrate the invention in detail, butwithout restricting it thereto.

EXAMPLE 1

Preparation of Pseudoionone

370 kg/h of citral (2.43 kmol/h), approx. 26 kg/h of recycled citral,3800 kg/h of aqueous 95% acetone and 30 kg/h of 5% aqueous NaOH aremixed. This forms a homogeneous solution which is passed through a phaseseparator as a precaution. When the water content or else the NaOHcontent is higher than specified, an aqueous phase can separate afterthe mixing process and has to be separated out. The mixture is heated to108° C. and pumped through a 160 l tubular reactor. The heat of reactionheats the mixture further to approx. 112° C. At a residence time ofapprox. 2 min, a conversion of approx. 93% is achieved.

The reaction mixture from the tubular reactor is decompressed tostandard pressure. In the course of this, approx. 2000 l/h of acetonedistill off, and the product solution cools to approx. 60° C.Subsequently, the mixture is freed of residual acetone in countercurrentwith approx. 700 kg/h of steam. Sufficient acetic acid is added to thesteam that the sodium hydroxide solution in the mixture is neutralizedand the effluent, aqueous mixture has a pH of 4-5. The acetone isrecycled back into the process.

Removal of the aqueous phase is followed by drying at approx. 100° C.and approx. 50 mbar and distillative purification in a dividing wallcolumn having 2 side draws. At the lower side draw, approx. 400 kg/h ofpseudoionone having a purity of 98% are obtained (GLC area %, sum of allisomers!). At the upper side draw, approx. 26 kg/h of citral areobtained (sum of all isomers) and are recycled continuously into theprocess.

EXAMPLE 2

Methylpseudoionones

110 kg/h of citral (0.72 kmol/h), approx. 20 kg/h of recycled citral(0.13 kmol/h), 1800 kg/h of aqueous 82% 2-butanone and approx. 20 kg/hof 5% aqueous NaOH are mixed. This forms a homogeneous solution which ispassed through a phase separator as a precaution. When the water contentor else the NaOH content is higher than specified, an aqueous phase canseparate after the mixing process and has to be separated out. Themixture is heated to 136° C. and pumped through a 160 l tubular reactor.The heat of reaction heats the mixture further to approx. 138° C. Withina residence time of 4 minutes, a conversion of approx. 82% based oncitral is attained.

Removal of the aqueous phase is followed by drying at approx. 100° C.and approx. 50 mbar and distillative purification in a dividing wallcolumn having 2 side draws. At the lower side draw, approx. 100 kg/h ofmethylpseudoionone having a purity of 98% are obtained (GLC area %, sumof all isomers!). At the upper side draw, approx. 20 kg/h of citral areobtained (sum of all isomers) and are recycled continuously into theprocess.

An iosmer ratio of about n: iso=5:1 is attained.

EXAMPLE 3

Methylpseudojonones

100 kg/h of citral (0.66 kmol/h), approx. 25 kg/h of recycled citral(0.16 kmol/h), 2200 l/h of aqueous 88% 2-butanone and approx. 120 kg/hof 40% aqueous NaOH are mixed. This forms a biphasic solution which ispassed through a phase separator. Approx. 120 kg/h of aqueous NaOH areremoved and approx. 2100 kg/h of organic phase are heated at 120° C. andpumped through a 160 l tubular reactor. The heat of reaction heats themixture further to approx. 132° C. At a residence time of 4 minutes, aconversion of approx. 75% based on citral is attained.

The reaction mixture from the tubular reactor is decompressed tostandard pressure. In the course of this, 1000 l/h of 2-butanone distilloff, and the product solution cools to approx. 75° C. Subsequently, themixture is freed of residual 2-butanone in countercurrent with approx.550 kg/h of steam. Sufficient acetic acid is added to the steam that thesodium hydroxide solution in the mixture is neutralized and theeffluent, aqueous mixture has a pH of 4-5.

Excess 2-butanone is recycled back into the process.

Removal of the aqueous phase is followed by drying at approx. 100° C.and approx. 50 mbar and distillative purification in a dividing wallcolumn having 2 side draws. At the lower side draw, approx. 100 kg/h ofmethylpseudoionones having a purity of 98% are obtained (GLC area %, sumof all isomers). At the upper side draw, approx. 25 kg/h of citral areobtained (sum of all isomers) which are recycled continuously into theprocess.

An isomer ratio of about n:iso=−8:1 is obtained.

EXAMPLE 4

Methylionones

Approx. 140 l/h of methylpseudoionones, 400 l/h of hexane (precooled to−8° C.!) and 200 l/h of 89% sulfuric acid are intimately mixedsuccessively in a reaction pump. The reaction mixture heatsspontaneously to approx. 29° C., and is cooled to about 26° C. andcirculated by pumping in a delay zone at this temperature for approx. 2min, before it is diluted with approx. 600 l/h of water in a furtherreaction pump. The mixture heats as a result of the addition of water toapprox. 47° C. and is kept at <45° C. using a downstream cooler. Afterthe water phase having a high sulfuric acid content is removed and,after further water scrubbing, the hexane is removed in countercurrentwith steam. The hexane is recycled back into the reaction.

Removal of the aqueous phase is followed by drying at approx. 50 mbarand 100° C. and purification in a dividing wall column having 2 sidedraws. In the main fraction (side draw 1), approx. 120 l/h ofmethylionones having a content of β-n-methylpseudoionone between 80 and90% are obtained.

The table which follows lists one of the typical compositions¹⁾:α-isomethylionone 8.8% ± 1   Sum of 10.7% ± 1.6 β-isomethylionone 0.8% ±0.1 isomethylionones γ-isomethylionone 1.1% ± 0.5 α-n-methylionone 0.25%± 0.1  Sum of 85.5% ± 1.8 β-n-methylionone 85.0% ± 1.5  n-methyliononesγ-n-methylionone 0.3% ± 0.2¹⁾ E- and Z-isomers are detected together, and the E-isomers dominate.

1. A continuous process for preparing pseudoionones of the generalformulae I or I′and isomers thereof

where R¹ is

R², R³ are each hydrogen, CH₃ or C₂H₅, R⁴, R⁵ are each hydrogen or CH₃,by reacting an aldehyde of the formula (II)

with an excess of a ketone of the general formula (III)

where R¹, R² and R³ are each as defined above, in the presence of waterand alkali metal hydroxide at elevated temperature in homogeneoussolution, which comprises a) mixing the homogeneous solution ofaldehyde, ketone and aqueous alkali metal hydroxide at a temperature offrom 10 to 120° C., then b) removing the water and alkali metalhydroxide which have not dissolved in the reaction mixture, c)subsequently passing the homogeneous reaction mixture, avoidingbackmixing, at a temperature which is from 10 to 120° C. above theboiling point of the lowest-boiling component and a vapor pressure p offrom 10⁶ to 10⁷ Pa through a reactor which enables a residence time offrom 2 to 300 minutes, d) cooling the reaction mixture underdecompression, e) removing the ketone from the reaction mixture withsteam in countercurrent and f) drying the crude product and freeing itof excess aldehyde and secondary components using a rectificationcolumn.
 2. The process according to claim 1, wherein the ketonecomponent of the general formula (II) is added in a from 5- to 50-foldmolar excess, and the unconverted proportion, downstream of the reactionzone, is removed at a pressure of from 10⁷ to 5·10⁸ mPa_(abs) and addedagain to the fresh ketone for the synthesis.
 3. The process according toclaim 1, wherein the reaction temperature at a given residence time isselected in such a way that the conversion of the aldehyde component isfrom 60 to 98%, and the unconverted aldehyde is removed and recycledinto the reaction.
 4. The process according to claim 1, wherein thewater content of the ketone, used for the reaction, of the formula (III)is between 1 and 15% by weight.
 5. The process according to claim 1,wherein the concentration of the alkali metal hydroxide used for thereaction is between 0.005 and 50% by weight.
 6. The process according toclaim 1 for preparing pseudoionones of the formula I and isomers thereofwhere R² or R³ is methyl, wherein the concentration of the alkali metalhydroxide used for the reaction is from 10 to 50% by weight.
 7. Theprocess according to claim 1, wherein the ketone of the formula (III)used consists substantially of excess ketone of the formula (III) whichhas been removed from the reaction and has a water content of 1-15% byweight, which may be supplemented with either anhydrous or aqueousketone of the formula (III) having a water content of 1-15% by weight.8. The process according to claim 1, wherein, in the case of reactionwith ketones of the general formula (III) where R²≠H and R³=H, a productmixture is obtained which contains from 70 to 95% n-alkylpseudoiononesand from 5 to 30% isoalkylpseudoionones


9. A continuous process for preparing ionones of the general formulae(IV), (V) and (VI) and isomers thereof, which comprises reacting thepseudoionones obtained according to claim 1 to give ionones of thegeneral formulae (IV) to (VI)

in the form such that the ratio of the n-form (R²=H, R³=alkyl) to theiso-form (R²=alkyl, R³=H) is maintained.
 10. The process according toclaim 9, wherein the pseudoionones are reacted with highly concentratedsulfuric acid in the presence of a diluent which is inert under thereaction conditions to give ionones, the reaction temperature being0-20° C. and the residence time between cyclization and hydrolysis beingfrom 10 to 300 seconds.
 11. The process according to claim 3, whereinthe water content of the ketone, used for the reaction, of the formula(III) is between 1 and 15% by weight.
 12. The process according to claim11, wherein the concentration of the alkali metal hydroxide used for thereaction is between 5 and 10% by weight.
 13. The process according toclaim 12 for preparing pseudoionones of the formula I and isomersthereof where R² or R³ is methyl, wherein the concentration of thealkali metal hydroxide used for the reaction is from 35 to 45% byweight.
 14. The process according to claim 13, wherein the ketone of theformula (III) used consists substantially of excess ketone of theformula (III) which has been removed from the reaction and has a watercontent of 1-15% by weight, which may be supplemented with eitheranhydrous or aqueous ketone of the formula (III) having a water contentof 1-15% by weight.
 15. The process according to claim 14, wherein, inthe case of reaction with ketones of the general formula (III) whereR²≠H and R³=H, a product mixture is obtained which contains from 70 to95% n-alkylpseudoionones and from 5 to 30% isoalkylpseudoionones


16. A continuous process for preparing ionones of the general formulae(IV), (V) and (VI) and isomers thereof, which comprises reacting thepseudoionones obtained according to claim 15 to give ionones of thegeneral formulae (IV) to (VI)

in the form such that the ratio of the n-form (R²=H, R³=alkyl) to theiso-form (R²=alkyl, R³=H) according to claim 15 is maintained.
 17. Theprocess according to claim 16, wherein the pseudoionones are reactedwith highly concentrated sulfuric acid in the presence of a diluentwhich is inert under the reaction conditions to give ionones, thereaction temperature being 0-20° C. and the residence time betweencyclization and hydrolysis being 120 seconds.